Techniques for COFDM DTV broadcasting are prescribed in the ETSI TS 302 755 V1.3.2 Technical Specification titled “Digital Video Broadcasting (DVB); Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2)” published in April 2012 by the European Telecommunications Standards Institute (ETSI). This DVB-T2 standard is based on time-division multiplex of T2 frames of COFDM symbols possibly having future-extension frames (FEFs) interleaved therewith, every six consecutive ones of which frames is considered to constitute a “super frame”. The duration of a T2 frame can be of as long a length as 250 ms (milliseconds) and begins with a P1 portion of a preamble, which P1 portion signals which of various ways for transmitting COFDM DTV signals is currently in use. The number of ways for transmitting COFDM DTV signals that can be signaled in each P1 preamble is 24, 8 in an S1 period and 16 in an ensuing S2 period. The three-bit S1 field indicates whether the currently incoming transmissions are T2 frames using 64,800-bit LDPC codeblocks, T2-lite frames using 16,200-bit FEC codeblocks, or FEFs using some still-to-be-decided form of FEC coding. The three-bit S1 field further indicates whether the P2 portion of the preamble following the P1 portion is to be interpreted presuming single-input/single-output (SISO) reception or multiple-input/single-output (MISO) reception. The first 3 bits of the 4-bit S2 field are referred to as S2 field 1. When the preamble format is of the type T2_SISO, T2_MISO, T2-LITE_SISO or T2-LITE_MISO, S2 field 1 indicates the inverse fast Fourier transform (I-FFT) size and provides partial information about the guard interval for the remaining symbols in the T2-frame. The ensuing single-bit S2 field 2 that concludes the S2 field indicates whether the preambles of the T2 frames in a super frame are all of the same type or not.
DVB-T2 permits the time-division multiplexing of several physical layer pipes (PLPs), and information concerning the operating characteristics of those PLPs is conveyed by first-layer (L1) signals that immediately follow the P2 portion of the preamble. Each L1 signal consists of an L1-pre signaling initial portion followed by an L1-post signaling portion. All L1 signaling data, except for the dynamic L1-post signaling, shall remain unchanged for the entire duration of one super-frame. Therefore, any changes implemented to the current configuration (i.e., the contents of the L1-pre signaling or the configurable part of the L1-post signaling) shall always occur at the transition from one super-frame to the consecutive next super-frame. The L1-pre signaling is 200 bits in size. It specifies whether the L1 signal is a repeat or not, the length of guard intervals, the type of peak-to-average-power ratio (PAPR) reduction employed, the natures of FEC coding and the modulation of OFDM carriers in the ensuing L1-post signaling, the size of the ensuing L1-post signaling, pilot carrier pattern, whether L1-post signaling is scrambled or not, and other items related to super-frame structure. The L1-pre signaling concludes with 32 parity bits of cyclic redundancy check (CRC) coding. The so-called “configurable” portion of L1-post signaling is 257 bits in size and comprises information relating to FEFs and to PLPs. The “configurable” field can be followed by “dynamic” and “extension” fields. The L1-post signal concludes with 32 parity bits of CRC coding followed by bits for padding the L1-post signal to prescribed size. The complete L1 signal includes 457 bits for signaling. Nonetheless, additional signaling capability may be required, and per custom this has been afforded by so-called “in-band” signaling wherein control signals replace portions of the broadcast normally allocated to DTV signal.
DVB-T2 was designed to supplant an earlier DVB-T standard for terrestrial DTV broadcasting, as specified in the ETSI TS 300 744 V1.5.1 Technical Specification titled “Digital Video Broadcasting (DVB); Frame structure channel coding and modulation for digital terrestrial television” published in April 2004 by ETSI. In DVB-T some of the continual pilot carriers were used to convey transmission-parameters signaling (TPS), and are referred to as “TPS carriers”. DVB-T2 dispensed with TPS carriers conveying transmission-parameters signaling in the time domain in favor of OFDM frame preambles conveying transmission-parameters signaling in the frequency domain. This reduced the likelihood of loss of a few TPS bits owing to short-duration drop-outs in received signal strength that might occur at any time in an OFDM frame, particularly during reception by a DTV receiver in a moving vehicle. On the other hand, a short-duration drop-out in received signal strength will occasionally occur at the beginning of an OFDM frame, resulting in the loss of all TPS information regarding the frame. This is not of too much concern if all OFDM frames share the same TPS information. Also, signaling can be provided relating the TPS information concerning each OFDM frame to similar TPS information conveyed in the preamble of at least one other OFDM frame.
The Advanced Television Systems Committee (ATSC) is an international consortium of television broadcasters, manufacturers of equipment for transmitting DTV signals, and manufacturers of equipment for receiving DTV signals. ATSC has sought to develop a “universal” standard for terrestrial over-the-air COFDM DTV broadcasting that will continue to be used for several years, which standard is referred to as ATSC 3.0. However, a concern of the broadcasting community as expressed in ATSC by Arthur Allison is that the development of future standards for terrestrial over-the-air COFDM DTV broadcasting be accommodated by transmitters specifically signaling receivers as to whether they transmit COFDM TV according to ATSC 3.0, some later version of ATSC 3.0 or some future broadcasting standard. Increasing the number of bits in the “configurable” portion of L1-post signaling to accommodate additional parameters descriptive of some later version of ATSC 3.0 or some future broadcast standard undesirably increases signaling overhead.
A better approach is to encode a digital signature sequence that is the key for specific interpretation of further bits of signaling in an L1 signal. This facilitates one of a plurality of different broadcast standards using some or all of the further bits in an L1 signal differently from another or others of that plurality of different broadcast standards. It is desirable that a number of digital signature sequences can be readily distinguished each from all others, but this undesirably tends to require quite a few bits for each digital signature sequence, which also increases signaling overhead.
Furthermore, it is desirable that COFDM transmission of a digital signature sequence be very robust, since several signaling parameters are apt to be affected in transition from one DTV broadcast standard to another. Providing this robustness for the COFDM transmission of the digital signature sequence undesirably tends to increase signaling overhead still further.
ATSC has also sought to develop a “universal” standard for terrestrial over-the-air COFDM DTV broadcasting in most countries in the world, even though these countries prescribe a variety of permissible radio-frequency channels for such broadcasting. These RF channels have different bandwidths in different countries, six megahertz (MHz) bandwidth being used in North American countries and 8 MHz bandwidth being used in European and Asian countries. Manufacturers of equipment for receiving DTV signals are apt eventually to desire development of a bandwidth-agnostic way to specify the nature of RF channels selected for reception. A bandwidth-agnostic way of specifying the nature of RF channels selected for reception will allow portable DTV receivers to continue to be able to receive DTV signals usefully when transported from a country in which 6 MHz RF bandwidths are used in over-the-air DTV broadcasting to a country in which 8 MHz RF bandwidths are used for over-the-air DTV broadcasting, or vice versa.
Accordingly, in early 2015 ATSC decided to adopt a method for transmitting certain metadata, which method ATSC members refer to as “bootstrap” signaling. This method is described in detail in ATSC Candidate Standard document “A/321 Part 1: ATSC Candidate Standard: System Discovery and Signaling” approved 6 May 2015. In this method “metadata” OFDM frames of bootstrap signaling are interspersed among the “full-bandwidth” OFDM frames conveying coded data and conveying other metadata in their respective preambles. The set of OFDM carriers in the metadata OFDM frames of bootstrap signaling is confined to a 4.5 MHz midband portion of the RF channel, facilitating its use with a variety of RF channel bandwidths and specifying use of 6 MHz, 7 MHz, 8 MHz or wider than 8 MHz RF channels. The metadata OFDM frames of bootstrap signaling use a set of OFDM carriers that are apt to have frequencies different from the set of OFDM carriers in the full-bandwidth OFDM frames used for conveying coded data. However, the spacing between OFDM carriers in the full-bandwidth OFDM frames used for conveying coded data is in a specified ratio with the spacing between OFDM carriers in the metadata OFDM frames of bootstrap signaling. This ratio is an important one of the transmission parameters specified in the bootstrap signaling.
The values used for each bootstrap symbol originate in the frequency domain with a 1449-sample Zadoff-Chu (ZC) sequence modulated by a pseudo-noise (PN) sequence. The chips of the PN sequence each have the same duration as a lobe of the ZC. This allows the ZC-root and PN-seed to signal respectively the major and minor versions of a broadcast service independently of each other. The successive complex samples of the resulting sequence are applied per respective OFDM carrier at the IFFT input. The PN sequence introduces a phase rotation to individual complex subcarriers, thus retaining the desirable Constant Amplitude Zero Auto-Correlation (CAZAC) properties of the original ZC sequence. The PN sequence further suppresses spurious peaks in the autocorrelation response, thereby providing additional signal separation between cyclic shifts of the same root sequence from one bootstrap symbol to the next. Both the ZC sequence and the PN sequence have reflective symmetry about the DC subcarrier. Consequently, the product of these two sequences also has reflective symmetry about the DC subcarrier.
Each metadata frame consists of a number of successive bootstrap symbols in the frequency domain. The initial one of these bootstrap symbols conveys the ZC-root as modulated by the PN seed. Succeeding bootstrap symbols transmit information by rotation of the circular 2K I-FFT from its previous position. The circular I-FFT of the concluding bootstrap symbol in each metadata COFDM frame is rotated one-half revolution respective to the circular I-FFT of the preceding bootstrap symbol to signal the conclusion of the metadata frame.
The metadata conveyed in a bootstrap symbol are susceptible to loss if they are transmitted at a time in which the DTV receiver experiences a momentary drop-out in received signal strength. So are the metadata conveyed in the preamble of a “full-bandwidth” OFDM frame. Occasional loss of metadata owing to momentary drop-outs in received signal strength would be less of a problem if the metadata were transmitted more than once during the course of each OFDM frame. Transmission of back-up metadata in OFDM symbol intervals to some extent undesirably reduces effective code rate for digital payload.
The inventors observe that transmission of back-up metadata by modifying the binary phase-shift keying (BPSK) of the continual pilot carriers does not reduce effective code rate for digital payload. While the DVB-T practice of transmission parameter signaling (TPS) by BPSK of continual pilot carriers was discarded in the newer DVB-T2 standard, the inventors point out that the problem with the TPS pilot carriers prescribed in the DVB-T standard was in substantial part because the TPS was transmitted in accordance with the DVB-T standard without sufficient redundancy to overcome momentary drop-outs in received signal strength.
The inventors point out that, while introducing redundancy into the digital signal for modulating the continual pilot carriers reduces the number of bits that can be transmitted in such BPSK signal, the reduced number of bits can be a signature of the DTV transmission standard in use. The inventors further point out that such signature can then be used as read addressing for a read-only memory (ROM) implementing a look-up table (LUT) for TPS signaling that has many more bits than does the signature used as read address. TPS signaling from the LUT can comprise a sufficient number of bits to describe optimal selections of many transmission parameters.
The inventors also point out that the signature can be used as partial read addressing for a ROM receiving metadata from the bootstrap signal and from the preambles of full-bandwidth OFDM frames as further partial read addressing. This ROM can be used as a LUT for TPS signaling to control structuring and operation of the DTV receiver. The signature metadata conveyed by the BPSK of the continual pilot carriers can be used to change the “dictionary” for the metadata from other sources. Such procedure can greatly extend the signaling capabilities of the first-layer (L1) bits in the preambles of OFDM frames, for example. This reduces any need for “in-band signaling” in the OFDM symbols following those preambles.
Many experts in digital communications strongly tend to favor coding being used to provide redundancy in digital transmissions, since coding is less affected by precisely when drop-outs in received signal occur than repeated transmissions of the same data tend to be. Error-correction coding (ECC) of TPS results in coded metadata having a similar problem to the uncoded metadata insofar as BPSK of continual pilot carriers is concerned. There is a tendency for peak-to-average power ratio (PAPR) of the continual pilot carriers to vary considerably at times being larger than desirable. The inventors favor the metadata being transmitted by phase shift of a repetitive sequence having reasonably constant PAPR, the length of each cycle of such repetitive sequence extending over a small enough number of OFDM symbol intervals that the sequence can be cyclically repeated a few times in an OFDM frame interval.